class Solution {
public:
    
    int dp[20];
    
    int numTrees(int n) {
        dp[0] = 1;
        dp[1] = 1;
        
        for (int i = 2; i <= n; i ++ ) 
            for (int j = 1; j <= i; j ++ ) 
                dp[i] += dp[j - 1] * dp[i - j];
        
        return dp[n];
    }
};
